Glass fibers are typically protected from external forces with one or more coating layers. Typically, two or more layers of coatings are applied during the optical-fiber drawing process (i.e., whereby a glass fiber is drawn from an optical preform in a drawing tower). A softer inner coating layer typically helps to protect the glass fiber from microbending. A harder outer coating layer typically is used to provide additional protection and to facilitate handling of the glass fiber. The coating layers may be cured, for example, using heat or ultraviolet (UV) light.
UV curing requires that the coated glass fiber be exposed to high intensity UV radiation. Curing time can be reduced by exposing the coating to higher intensity UV radiation. Reducing curing time is particularly desirable to permit an increase in fiber drawing line speeds and thus optical-fiber production rates.
Mercury lamps (e.g., high pressure mercury lamps or mercury xenon lamps) are commonly used to generate the UV radiation needed for UV curing. One downside of using mercury lamps is that mercury lamps require a significant amount of power to generate sufficiently intense UV radiation. For example, UV lamps used to cure a single coated fiber (i.e., one polymeric coating) may require a collective power consumption of 50 kilowatts.
Another shortcoming of mercury lamps is that much of the energy used for powering mercury lamps is emitted not as UV radiation, but rather as heat. Accordingly, mercury lamps must be cooled (e.g., using a heat exchanger) to prevent overheating. In addition, the undesirable heat generated by the mercury lamps may slow the rate at which the optical fiber coatings cure.
Furthermore, mercury lamps generate a wide spectrum of electromagnetic radiation, such as having wavelengths of less than 200 nanometers and greater than 700 nanometers (i.e., infrared light). Typically, UV radiation having wavelengths of between about 300 nanometers and 400 nanometers is useful for curing UV coatings. Thus, much of the electromagnetic radiation generated by mercury bulbs is wasted (e.g., 90 percent or more). Additionally, glass fibers possess an exemplary diameter of about 125 microns, which, of course, is much smaller than the mercury bulbs. Consequently, most of the UV radiation emitted by the mercury lamps does not reach the glass fiber's uncured coating (i.e., the energy is wasted).
It may thus be advantageous to employ UVLEDs, an alternative to conventional mercury lamps, to cure glass-fiber coatings. UVLEDs typically require significantly less energy and correspondingly generate much less heat energy than conventional UV lamps.
For example, U.S. Pat. No. 7,022,382 (Khudyakov et al.), which is hereby incorporated by reference in its entirety, discloses the use of UV lasers (e.g., continuous or pulsed lasers) for curing optical fiber coatings.
U.S. Patent Application Publication No. 2003/0026919 (Kojima et al.), which is hereby incorporated by reference in its entirety, discloses the use of ultraviolet light emitting diodes (UVLEDs) for curing optical fiber coatings. The disclosed optical fiber resin coating apparatus includes a mold assembly in which a UV curable resin is coated onto an optical fiber. Also at the mold assembly, the coated optical fiber is exposed to UV radiation from a number of UVLEDs to cure the UV coating. A control circuit may be used to control the UV radiation output from the UVLED array. For example, the control circuit may reduce the current to one or more UVLEDs to reduce the intensity of emitted UV radiation. The control circuit may also be used to vary the intensity of the UV radiation as the optical fiber progresses through the mold assembly.
Even so, UVLEDs, though more efficient than mercury lamps, still waste a significant amount of energy in curing glass-fiber coatings. In particular, much of the emitted UV radiation is not used to cure the glass-fiber coatings.
Therefore, a need exists for a UVLED apparatus that, as compared with a conventional mercury-lamp device, not only consumes less power and generates less unwanted heat, but also is capable of curing glass-fiber coatings with improved curing efficiency.